1. Field of the Invention
The present invention relates to attenuated total reflection (ATR) infrared chemical imaging, and more specifically, it relates to method for correcting the image collected using ATR through a germanium hemisphere and one dimensional linear array detector.
2. Description of Related Art
The conventional ATR infrared chemical imaging system utilizing a Ge hemisphere provides chemical information abstracted in the form of 2D or 3D color graphics from the infrared spectra of heterogeneous samples. It obtains infrared spectra from a wide area of a sample from square segment-to-segment areas with a regular segment size in 2 dimensional directions, using a Fourier transform infrared spectrometer (FT-IR) combined with an optical microscope. Instead of using one detector, a one or two dimensional array detector obtains infrared spectra from multiple positions in order to shorten the measurement time. Contents of certain infrared information of every sample segment are converted to a false color and displayed as a full color graph.
Often, infrared ATR imaging systems use a Ge hemisphere to focus the infrared incident beam at the sample position, so that the incident beam is focused at the sample and the infrared beam is totally reflected back from the sample-Ge boundary. The reflected beam is detected by a detector or a linear or 2 dimensional array detector. The use of an array detector reduces the required measurement time because an array detector is composed of multiple detectors. The reflected beam carries the spectral information of the sample. The infrared Ge ATR imaging technique is a popular technique to study fine structures that have a size that is on the order of a few micron, because the refractive index of Ge (n=4) provides additional magnification above the nominal magnification of infrared microscopes. For instance, if an infrared microscope has adequate sensitivity to obtain spectra from a small area such as a 10×10 micron area, the Ge ATR technique may enable the collection of spectra from areas such as a 2.5×2.5 micron sample area. Thus, instead of generating a chemical image of a sample by composing it with 10 micron portions of the sample area, it is possible to obtain the chemical image by stitching observed data for 2.5 micron steps. With the anticipation of this high resolution ability, thin adhesive layers of polymer laminate films, where each film is about 4 microns thick, have been analyzed and displayed as chemical images. There are, however, at least three major problems in this technique as discussed below.
By utilizing the Ge hemisphere as an additional lens in an infrared microscope system, spectroscopic measurements are affected by optical aberrations and distortion. Ge ATR imaging measurements show significant pincushion type distortion in displayed chemical images. FIG. 1A shows a chemical image of a sample prepared as a regular 2-dimensional array of squares. The sample, to illustrate the workings of the embodiments herein, is a polyethylene terephthalate film embossed with a regular pattern. The dotted line denoted by the reference character 101 shows the degree to which the upper row of squares is distorted by a Ge hemisphere ATR technique. This effect, as known to those of ordinary skill in the art of optics, resembles pincushion distortion. However, the distortion typically provided Ge hemisphere ATR analysis is not necessarily axially symmetric the optical system often includes off-axis elements. The blurred images of square objects in the left and right sides of bottom corners in FIG. 1A reveals the presence of additional aberrations. Further detailed studies of small areas show the abrupt changes in distortion when an example Ge/sample unit is moved by, for example, a distance of 16 pixels along the x-axis, i.e., the direction of the long axis of the array, as shown in FIG. 1B. The discontinuities, as illustrated by arrow 10 and arrow 12 in FIG. 1B, appear every 16 pixels as distinguished by denoted line 14 and denoted line 16. All those distortions give rise to an incorrect chemical image of the sample.
Even when a Ge hemisphere is located in the center of the infrared beam, maintaining the optical symmetry to provide efficient infrared beam throughput is different from sample segment-to-segment with each segment being a desired measurement point. In addition, when the Ge hemisphere and the paired sample are moved together to change the measurement locations, loss of optical symmetry often further changes optical throughput. FIGS. 2A, 2B and FIG. 3A and FIG. 3B are thus provided to illustrate such typical changes in ATR optical throughput. In particular, the optical throughput of an entire microscope and Ge hemisphere system was measured as single beam map for a sample area of 3.2 mm(x)×4 mm(y). FIG. 2A shows an example plot of overall optical throughput as a measurement of infrared intensity of an individual segment at 2,000 cm−1. These data were collected in 6 passes of a 16-element array detector across the sample. The dark bar 201 shown in FIG. 2A is provided to illustrate the approximate width of the utilized array detector, and represents the width of a single pass. In essence, FIG. 2A illustrates the triple dependence of optical throughput of such a system on: 1) Position along the array detector; 2) Displacement of the Ge hemisphere in the direction of the array detector; 3) Displacement of the Ge hemisphere in the direction perpendicular to the array detector. FIG. 2B shows a corresponding plot of the cross section of the graph shown in FIG. 2A. Regions 2b1-2b6, separated by dashed lines, show the 6 passes of the detector. Denoted line 207 shows an overall quadratic dependence of intensity with position along the axis of the detector. FIG. 3A shows the same cross section, with the regions 2b1-2b6 again shown separated by dashed lines, and further illustrates a quadratic fitted dependence via the denoted fits 307 along the correlated detector widths itself. FIG. 3B shows the same cross-section, with the regions 2b1-2b6 once again shown separated by dashed lines, after both quadratic functions is removed, leaving the sensitivity of individual pixels as the only remaining source of variation. Since the IR spectrum must be calculated as single beam spectra of sample divided by matched incident beam, measurements of a sample map and an incident beam map are recommended. This requires long measurement times compared with usual transmission or reflection measurements, in which incident beam spectra are taken only at one position. In addition, regions of the map with low throughput results in a deleterious degradation of signal-to-noise due to decreased intensity of the incident beam map.
The incident angle is different from sample segment to sample segment. Different incident angles result in different measurement conditions for each ATR spectrum. Thus, each spectrum has different observation characteristics and cannot be equivalent to the others. In spectroscopic determinations of the properties of samples, every spectrum must have the same observation characteristic except for the properties of the sample. Thus, a chemical image deduced from the Ge ATR method has certain errors due to the non-uniform incident angle distribution.
It is therefore desirable to correct (1) distortion and aberrations, (2) location dependent optical throughput and (3) location dependent incident angle in the chemical images deduced from the Ge hemisphere ATR method. The present invention provides such corrections.